The present invention relates to a dual polarisation antenna adapted to reflect electromagnetic waves of two orthogonal polarisations with low radiation side lobes.
The present antenna can more particularly be utilised in a satellite radiocommunication system that employs orthogonal polarisation, one polarisation being horizontal and the other vertical.
The technique is well known in the art, for instance in U.S. Pat. No. 4,757,323, which is incorporated herein by reference. It discloses how to manufacture a dual polarisation same-zone two-frequency antenna for telecommunications satellites. The antenna serves to focus and to direct electromagnetic energy along a communication link.
The antenna has a reflector and a source that is of the horn type. The reflector reflects two electromagnetic waves which are polarised orthogonally to each other and which are at different frequencies in such a manner as to obtain the some geographical coverage on the surface of the globe.
The central portion of the reflector, constituted by the area common to both first and second orthogonal grids, reflects both orthogonally polarised waves; whereas the peripheral portion outside the central second grid only reflects the low frequency polarised wave. The same zone coverage is obtained by determining the area and shape of the central grid in such a manner as to obtain the some zone coverage with the high frequency wave as is obtained by the first grid for the low frequency wave.
It is usually necessary to preserve a direction of polarisation of the electromagnetic energy, and to prevent the generation of both grating and side lobe components that produce interference on said desired direction of polarisation and in the rest of the satellite useful payload.
It is a disadvantage of prior art techniques that undesired grating lobes in the radiation pattern, namely in the desired direction, of an antenna are generated by antenna with many antenna elements, e.g. with several antenna elements. Further, grating lobes are undesired side lobes in the radiation pattern of an antenna.
On the other hand, wherein two reflectors are to be employed for the reflection of electromagnetic waves of differing polarisation, it is desirable to construct a single supporting structure for both reflectors, thereby conserving overall weight of the antenna. Such a sharing of support structure requires a positioning of a source such that its respective polarised electromagnetic waves impinge upon the desired reflectors for directing the waves of the respective polarisations in the desired directions.
In addition, the assembly of reflectors must be configured in a fashion such that the presence of one reflector does not interfere with the propagation of electromagnetic energy between a second reflector and the source associated therewith.
A problem arises in that constructional methods presently available for a composite antenna structure having plural reflectors entail a greater weight for the support structure than is desirable.
In brief, the prior art presents a central zone with two grids to reflect two orthogonal polarisations on different frequency, respectively, for the some coverage zone, which presents both grating and side lobes arising from the reflection of the orthogonal polarisations on the double grid. Likewise, a reflection zone formed by a single grid also favours the formation of side lobes that can have a harmful effect on the rest of the satellite useful payload.
There is therefore a need to develop a dual polorisation antenna with reduced grating and side lobes. The dual polarisation antenna has a central reflection zone for reflecting two orthogonal polarisations on different frequency and polarisation, respectively, for the same coverage zone, and a peripheral zone that reflects one polarisation and is transparent to its orthogonal polarisation. The antenna must be mounted on a single structure in order to reduce the overall weight of the assembly.
To overcome the disadvantages mentioned above, the present invention provides a dual polarisation antenna with low image lobes and grating lobes, which is capable of being used in radiocommunication system.
The antenna mentioned comprises a reflector assembly illuminated by a feed source of the horn type, for example. The reflector assembly comprises a front reflector that is adopted for reflecting two orthogonally polarised electromagnetic waves and each one on a different frequency and polarisation; an auxiliary reflector that is adopted for reflecting one of the arthogonally polarised electromagnetic waves; and a deflecting surface that is adapted for diffracting the orthogonally polarised electromagnetic wave that passes through the auxiliary reflector.
The front reflector is a continuous metallic surface deposited on a supporting surface, which is capable of reflecting two orthogonal polarisations without permitting the formation of side lobes that could produce interference on the rest of the satellite useful payload and without dissipating thermal energy in the reflector. RF losses are reduced at the front reflector due to that this reflector is continuous.
The auxiliary reflector is formed by a set of uniformly spaced metallic wires, positioned to form a ring around the frontal reflector, with the purpose of permitting the reflection of the polarisation parallel to the wires, and filtering the wave polarised orthogonally thereto.
The electromagnetic wave cited is deflected by the associated deflecting surface, which deflects totally the orthogonal polarisation that traverses the auxiliary reflector into free space, permitting the formation of side lobes to be minimised.
The antenna is equipped with a single grid that simplifies its construction. The thermo-elastic behaviour of the reflector assembly is more appropriate as it admits strengthening at will.